Method for the detection of impedances and for the qualification of the telephone lines

ABSTRACT

The invention relates to a method for detection of impedances, in particular along inductances, in telephone lines of the type with two metal wires as signal conductors (twisted pair), having the following method steps:
     a test signal in the form of an AC voltage is fed into the telephone line,   a measurement signal of the reflection signal of the test signal is measured, which can be tapped off the input impedance of the entire line at the start of the line,   the first method steps are carried out at a number of different frequencies within a preselected frequency range of the AC voltage of the test signal,   the profile of the measurement signals is analyzed as a function of the frequency, with   the derivative of the profile of the measurement signals being formed based on the frequency,   at which point the second derivative of the profile of the measurement signals is formed based on the frequency,   the profile of the second derivative of the profile of the measurement signals based on the frequency is investigated for one or more mathematical sign changes.   

     The invention also relates to a method for qualification of telephone lines of the type with two metal wires as signal conductors (twisted pair) for suitability for data transmissions based on the DSL Standard, and to use of a DSL modem for carrying out methods such as this.

The invention relates to a method for detection of impedances, inparticular serial inductances, in telephone lines with two metal wiresas signal conductors, for qualification of telephone lines with twometal wires as signal conductors (twisted pair) for suitability for datatransmissions based on the DSL Standard according to theprecharacterizing clause of Claim 1, and to the use of a DSL modem forcarrying out a method such as this.

In modern data transmission, which is used increasingly frequently andover ever larger areas, via conventional metallic telephone lines withtwo line cores (which are generally formed from copper wires), oneproblem that arises is that these lines, which were often laid decadesago, were not designed for transmission frequencies above 6 kHz.

Particularly in rural areas and in particular in the American area,lines have often been laid which were provided with so-called “loadcoils” in order to improve the transmission of frequencies in the rangefrom 1 to 5 kHz. These are series inductances which were looped in pairsinto the two line cores—provided with a common toroidal core—at regularintervals, for example with 66 mH in each case at intervals of 900meters, or with 88 mH in each case at intervals of 1.2 km.

However, transmission frequencies above 5 kHz, in the range from several10 to 100 kHz, must be possible for data transmission.

This is impossible in the presence of impedances, in particular theinductances which have been mentioned, whose purpose was to reduce theattenuation in the speech band, since they represent an excessively highimpedance for high frequencies.

Since there are often no accurate records relating to the type of lines,or to whether inductances were or were not laid, the line must bequalified before it can be used for data transmission.

This is expensive and highly time-consuming, particularly when telephonecompany employees have to be sent out in order to measure the line.

U.S. Pat. Nos. 5,465,287 and 4,620,069 disclose methods fordetermination of line impedances, which are preferably carried out indigital switching centers for telephone networks. Further evaluationmethods for determination of pole position and zero position frequencyinformation, which make it possible to deduce that there are inductanceson two-wire telephone lines, are described in U.S. Pat. Nos. 4,229,626and 4,307,267. The methods according to the prior art have the primarydisadvantage that it is either necessary to obtain complex additionaldevices, or the qualification of the respective telephone line cannoteasily be carried out by the subscriber himself.

The object of the invention is to provide a method in which anyimpedances for qualification of a conventional telephone line can bedetected at as low a cost as possible and with a high degree ofreliability.

This object is achieved by a method for qualification of telephone linesaccording to Claim 1, and by use of a DSL modem according to Claim 11and according to Claim 12.

The invention provides a method for detection of impedances, inparticular along inductances (looped-in in series), in telephone lineswith two metal wires as signal conductors in order to qualify them fortheir suitability for data transmission at frequencies above the speechband, having the following steps:

-   -   (a) a test signal in the form of an AC voltage is fed into the        telephone line,    -   (b) a reflection signal of the test signal is measured, which        can be tapped off as the component, reflected on the input        impedance of the entire line, of the test signal fed in at the        start of the line,    -   (c) the first two method steps (a) and (b) are carried out at a        number of different frequencies within a preselected frequency        range of the AC voltage of the test signal in order to measure        any phase shift in the reflection signal with respect to the        test signal at the respective frequency,    -   (d) the phase shift is analyzed as a function of the frequency        in order to assess the telephone line, in which case:        -   the derivative of the phase shift is formed on the basis of            the frequency,        -   the second derivative of the phase shift is formed,        -   the second derivative of the phase shift is investigated for            one or more mathematical sign changes, and    -   in which case        -   when a mathematical sign change occurs in the second            derivative of the phase shift, the telephone line is            assessed as not being suitable for use for data transmission            at frequencies above the speech band, without further            technical actions.

The invention proposes that an AC voltage signal be fed in, which is, ofcourse, partially reflected on the overall input impedance of the line.This reflected signal is then investigated for its line resistance. Thephase shift is recorded as a measurement signal of the reflection signalwith respect to the test signal. Investigation of the profile of thesecond derivative results in clear information as to whether there isany impedance in the line. This represents a considerable simplificationin comparison to previous measurement methods, having a number of teststeps which have to be carried out individually and manually.

This is used for qualification of telephone lines of the type with twometal wires as signal conductors (twisted pair) for suitability for datatransmissions based on the DSL Standard. When there is a mathematicalsign change in a second derivative of the phase difference between themeasurement signal and the test signal over the frequency in apreselected frequency range, the line is assessed as not being suitablefor use for data transmissions based on the DSL Standard, withoutfurther technical actions.

One preferred method step provides for the AC voltage to be a sinusoidalAC voltage. A sinusoidal AC voltage such as this can be generated anddetected easily on a DSL modem card.

According to one advantageous refinement of the method, the phase shiftis determined by means of a phase discriminator.

According to one refinement of the invention, which is likewiseadvantageous, the phase shift is determined by means of a quadraturedemodulator.

One advantageous method step provides for the frequencies to be chosento be between 1 and 5 kHz, in particular with regular or logarithmicintervals between the individual frequencies. The so-called load coilscan be detected particularly well, especially in this frequency range.

One particularly advantageous and thus preferred method step providesthat, before the second derivative of the profile of the measurementsignals based on the frequency is formed, the individual measurementsignals are averaged in order to smooth them in the profile. Thesmoothing is used to reduce the “noise components” (which arestatistically independent with respect to the actual profile), andimproves the capability to evaluate the data.

In consequence, median formation is carried out as the smoothing processaccording to one refinement of the invention.

One advantageous refinement of the method provides that, in a step whichfollows the median formation, individual smoothed measurement signals,which are at a regular interval from one another, are supplied forfurther evaluation. This leads to data reduction, which simplifies theevaluation process, and which does not result in any corruption of theresults, owing to the previous smoothing of the data.

A further aspect of the invention provides for use of a DSL modem forcarrying out the method as described above, using the data driver andreceiving module which is provided in the DSL modem that is used. Thismakes it possible to use already existing hardware in a particularlysimple manner, without any need for further developments.

Furthermore, the invention proposes the use of a DSL modem at thenetwork provider end for carrying out the method mentioned above, inwhich case the test module which is present in the DSL modem that isused at the switching center end is used, which is often provided in aDSL modem at the network operator end, in order to make it possible topass analog currents and/or voltages of different types to the line, andto measure them, in order to make it possible to carry out an electricaltest on the line in this way. In this case as well, the advantagesresult from the use of already available hardware.

Further advantages, special features and expedient developments of theinvention result from the further dependent claims or fromsub-combinations of them.

The invention will be explained in more detail in the following textwith reference to the drawing, in which:

FIG. 1 shows a detail of a telephone line with load coils,

FIG. 2 shows an equivalent circuit of the line for low frequencies,

FIG. 3 shows an equivalent circuit of the entire line with load coilsfor low frequencies,

FIG. 4 shows the qualitative profile of the characteristic impedance Zas a function of the frequency,

FIG. 5 shows the overall input impedance of the line plotted against thefrequency,

FIG. 6 shows the real part of the input impedance of the line plottedagainst the frequency,

FIG. 7 shows the imaginary part of the input impedance of the lineplotted against the frequency,

FIG. 8 shows the phase shift of the input impedance of the line plottedagainst the frequency,

FIG. 9 shows the first derivative of the phase shift of the inputimpedance as a function of the frequency for different constraints,

FIG. 10 shows the second derivative of the phase shift of the inputimpedance as a function of the frequency for different constraints,

FIG. 11 shows a flowchart of the method,

FIG. 12 shows a measurement example of a measured and calculated firstderivative,

FIG. 13 shows a measurement example of a measured and calculated firstderivative after averaging for smoothing,

FIG. 14 shows a measurement example of a measured and calculated firstderivative after data reduction has been carried out,

FIG. 15 shows a measurement example of a calculated second derivative,

FIG. 16 shows a schematic illustration of assemblies of a DSL modem,

FIG. 17 shows a schematic illustration of those assemblies of the DSLmodem which are involved in the evaluation process,

FIG. 18 shows a module which is involved in the analysis based on afirst example,

FIG. 19 shows a module which is involved in the analysis based on asecond example,

FIG. 20 shows a schematic illustration of the DSL modem assemblies whichare involved in the evaluation process for phase difference measurement,and

FIG. 21 shows a schematic illustration of signal profiles for phasedifference measurement.

Identical reference symbols in the figures denote identical elements orelements having the same effect.

FIG. 1 shows a longitudinal detail of a telephone line from the start(feed points 11 and 12 for the two individual wires 13 and 14) of theline 10. Without the load coils 15 and 16 inserted in it, thecharacteristic impedance of the line is Zo.

The load coils in the example are looped-in in series (along the line)in the line at a distance of 2 km from the feed point, and thenrepeatedly after every 2 km. The coils are designed such that theyreduce the line attenuation for frequencies in the speech band up to 3.4kHz. However, at higher frequencies, the attenuation rises drastically,so that data transmission is impossible with all DSL methods.

It is thus necessary to use methods described here to determine whetheran existing line is or is not provided with load coils, in order todetermine its suitability for transmission methods which useconsiderably higher transmission frequencies (for example ISDN, VDSL,SDSL, ADSL).

The methods according to the invention allow such determination ofsuitability in the sense of the presence or absence of load coils in theline, without any additional test equipment, using solely the existinghardware together with associated software.

FIG. 2 shows the equivalent circuit of the line arrangement shown inFIG. 1 for low frequencies, for which the line length as far as the coil(in the example 1=2 km) is very much shorter than the wavelength.

For simple estimation purposes, the line elements of the pure wire linecan be combined to form concentrated elements, specifically to form aresistor 21 (R′), the coil 22 (L′) and the capacitor 23 (C′).

The characteristic impedance of the line is changed from Zo to Zc by theincorporation of the load coils. The input impedance is obtained byterminating the equivalent circuit with the characteristic impedance Zc.

Zc takes account of the load coils.

Zc thus has a high resistive and capacitive component for lowfrequencies, as is shown in the further simplified equivalent circuit asshown in FIG. 3 by the switching element 30, which replaces the furtherline and the coils, with the resistor 31 (Rc) and the capacitor 32 (Cc).

The qualitative profile of the characteristic impedance plotted againstthe frequency w is shown in FIG. 4. The graph shows the real part of Zo41 and the real part of Zc 43 (that is to say when coils are present),as well as the imaginary part of Zo 42 and of Zc 44. The curves differnoticeably from one another. The difference will become even more clearin the following text. In this case, however, it can be seen that theprofile of the input impedance also has “ripples” in comparison to aline without load coils.

FIGS. 5 to 8 show the influence of constraints at the end 18 of a firstline section 13 composed of wire. Three curves are in each case plotted,with a denoting a value profile for a line which is open at the end 18,b denoting a value profile for a line which is connected at the end 18via a load coil to a further piece of line, and c denoting a valueprofile of a line which is connected at the end 18, without any coil,directly to a further piece of line.

FIG. 5 shows the magnitude of the input impedance of the line at thefeed point (11, 12). FIG. 6 shows the real part of the input impedance.FIG. 7 shows the imaginary part of the input impedance and, finally,FIG. 8 shows the phase shift of the input impedance of the line.

As can clearly be seen, the difference can be evaluated in all thevalues, but is strongest in the phase profile, which appears to suggestthat evaluation of the phase profile is preferable.

In this context, FIGS. 9 and 10 once again show the typical profile ofthe phase shift of the first derivative (FIG. 9) and of the secondderivative (FIG. 10) in more detail.

The described problem, that is to say the detection of the load coils,is solved by detecting the very different profiles of the characteristicimpedances between a line with and without load coils in the lowerfrequency range (that is to say in the speech band), to be precise usingthe already existing hardware.

FIG. 11 shows the procedure for the analysis part of the method after atest signal in the form of an AC voltage has been fed into the telephoneline and the phase shift of the reflection signal of the test signal hasbeen measured as the measurement signal at a number of differentfrequencies. For illustrative purposes, FIGS. 12 to 15 show theprocessing of the data records, once again with the boundary conditionsa, b and c (see above).

The analysis method steps are carried out as follows: Analysis of theprofiles of the measurement signals, with the derivative 91 of theprofile of the measurement signals based on the frequency being formed(see FIG. 12 for a typical data record). The profile measurement signalis then subjected to averaging 92 by forming the median of theindividual measurement signals in order to smooth their profile. In thiscase, by way of example, eight adjacent values may be smoothed jointly(see FIG. 13 for a typical data record).

Data reduction 93 is carried out in the step following the medianformation, in which only individual smoothed measurement signals, whichare separated by regular intervals (for example only every eighth value)are supplied for further evaluation (see FIG. 14 for a typical datarecord).

The second derivative 94 of the profile of the reduced smoothedmeasurement signals is now produced on the basis of the frequency (seeFIG. 15 for a typical data record).

All that now need be looked for is a mathematical sign change in theprofile of the second derivative (95). These exist in lines whichcontain load coils, but no mathematical sign changes occur in lineswithout load coils. The mathematical sign change can be used to clearlydeduce the presence (96) or absence (97) of load coils.

FIG. 16 shows a typical DSL module 100, as may be used. This has asine-wave generator 108, which supplies the signal via a transmissionfilter 105 a and the digital/analog converter 105 b to the hybrid 103(which also contains a line driver). The hybrid 103 is connecteddirectly to a transformer 104, via which the signal is fed into the line10 on both wires 13 and 14. The DSL module receives signals from theline 10 again via the transformer 104 and the hybrids 103, and suppliesthe separated signal via an analog/digital converter 106 b and areception filter 106 a to the echo compensation device 107. This isnormally used to actually separate its own reflected signal.

Some DSL cards 100 also have a line test device 102, as is illustratedin FIG. 16. This is able to pass analog signals (which are produced bymeans of signal generation devices 111 and 112) to the line 10, in orderin this way to carry out fundamental functional tests of the line. Forthis purpose, by way of example, measured values which are droppedacross resistors 113 and 114 are evaluated by means of an evaluationapparatus 115. The test may comprise simple resistance tests or the like(metallic loop test).

In order to carry out the method, the AM modulators in the transmissionpath can be used to produce the sinusoidal measurement signals. Thereception path comprises the ADC 106 b (analog/digital converter), thedownsampling from the ADC sampling rate to the symbol rate, the RXfilter 106 a and the echo compensation 107. The echo compensationcomprises the actual FIR echo compensator filter 107 and the adder 107a, which, in the data mode, subtracts the echo which is simulated by theecho compensator filter, from the filtered received signal (that is tosay switched off in the method). For adaptation, the remaining echo issupplied, downstream from the adder, to the adaptation part of the echocompensator filter. Furthermore, the reception path has an r*4 kHzdemodulator 107 b, by means of which the data can be recovered duringthe G.hs procedure.

The arrangement of the hybrid and transformer likewise corresponds tothe normal application. The transformer winding is split on the loopside, and the winding elements are connected to a capacitor in order toavoid a short circuit during power feeding. In this case, the hybridshould also contain the line driver, which may have an internalresistance Ri.

The method for detection in the transceiver will be described in thefollowing text: transmission of a sine-wave signal. TX and RX filtersconnected as bandpass filters. Echo compensation switched off, that isto say Ure=Ur. Demodulation of the “echo” and measurement of theamplitude of the demodulated signal.

In all of the measurements, the gain factors during transmission andreception and the internal resistance Ri remain the same. The voltage ofthe line start and thus also the complex value of the “echo” is obtainedfrom the voltage split between Ri and the complex Zc transformed via thetransformer and the hybrid. For lines which have load coils, the profileof the “echo” is different from that on lines which do not have loadcoils, and the demodulated signal is correspondingly different. It isthus possible to identify the presence of load coils from, for example,the profile of the demodulated signal.

The input resistance of the loop (line) is thus measured indirectly bymeasurement of the received signal. The relationship between thereceived signal and the transmission signal is measured as the transferfunction.

The line test device 102 likewise has line drivers which—controlled by“settings” by the HOST—can, for example, pass differential sine-wavetones to the line. The current can be measured at the driver outputs.

The following text describes how the method can be carried out using thetest device:

A differential sine-wave signal of constant amplitude is transmitted,and the amplitude of the driver current is measured. This is differentin the case of lines with load coils than in the case of lines withoutsuch coils, if the frequency is in a range in which the twocharacteristic impedances differ to a major extent (at low frequencies).It is thus possible to detect load coils.

Both specific methods are based on the assumption that the line is openat the end and is terminated by a telecommunications system which iscurrently not active, so that the input impedance of the line is not“corrupted” by a terminating impedance (which is generally in the regionof 135 ohms).

FIG. 17 once again shows the various areas in which the method iscarried out. First of all, the hardware which is used in DSL modems—asalready described—is used for measurement. The measurement signals 134and 135 which are tapped off can be evaluated both by software and byspecial hardware 131. The subsequent evaluation 132 of the analysisresults, which finally produces the result “load coils present/notpresent”, is generally in the form of software.

The input resistance of the line can be measured only indirectly withthe aid of the modem: in fact, the entire input impedance of the hybridis always measured. Since the transformer impedance has a very majoreffect on this, the difference in the magnitudes of the inputresistances of the hybrid are only very small between lines with orwithout load coils. It is very difficult to evaluate the measurementresults.

The detection of load coils can thus be carried out, in particular, bymeasurement of the profile of the phase of the input resistance of thehybrid in the frequency range from 1.5 to 5 kHz, and by determination ofthe gradients. The measurements could be carried out with a step widthof 100 to 200 Hz.

FIG. 18 and FIG. 19 show two different apparatuses for analysis of thephase profile, that is to say for formation of phase difference measuredvalues over the frequency. In the first variant (FIG. 18), themathematical sign (141, and 142) is in each case formed from thetransmission signal 134 and from the received signal 135, which aresinusoidal and have no DC voltage component, and are supplied to a(digital) phase discriminator 143. One specific embodiment relating tothis will be described further below (FIGS. 20 and 21).

The variant in FIG. 19 shows a quadrature demodulator 150 for formationof phase difference measured values, which carries out quadraturedemodulation of the received signal, with the transmission signal (testsignal) being used as the carrier.

FIG. 20 shows one embodiment for phase measurement using the modemhardware and a simple additional circuit. The corresponding signals areshown in FIG. 21. The production and TX filtering of the symbols arecarried out such that a sinusoidal transmission signal, without any DCvoltage component, is produced at frequencies from 1.5 kHz to 5 kHz.Corresponding to the voltage split between the line driver internalresistance and the hybrid input impedance, which includes the inputimpedance of the line 10, this results in a sinusoidal received signaldownstream from the analog/digital converter 106 b. If thediscrete-amplitude transmission signal and received signal are codedusing two's complement form, only the most significant bits (that is tosay the mathematical signs 210 and 220, which each have a flank 211, 212and 221, 222 when the mathematical sign changes) are in each casedetermined and passed on, by supplying them to an exclusive-NOR gate133. The output signal 230 with the corresponding flanks 231 and 232 ofthis gate 133 is filtered by means of a low-pass filter 131, whosecut-off frequency is, for example, 100 Hz. The output signal 240 fromthe low-pass filter is a measure of the phase difference between thetransmission signal and the received signal, and can be written for eachmeasurement to a register 132 which can be read by software. Theexclusive-NOR gate and the low-pass filter represent a simplecoincidence detector.

List of reference symbols  10 Line  11, 12 Feed points  13, 14 Wires 15, 16 Load coils  17 End of the line  18 End of the line section Zo,Zc Characteristic impedance  21 Resistance  22 Coil  23 Capacitor  30Switching element  31 Resistor  32 Capacitor  41 Real part of Zo  42Imaginary part of Zo  43 Real part of Zc  44 Imaginary part of Zc aValue profile of the open line b Value profile of the load coil c Valueprofile of the connected line  91 Derivative of the profile  92Averaging  93 Data reduction  94 Second derivative of the profile  95Search for a mathematical sign change  96 Mathematical sign change, yes 97 Mathematical sign change, no 100 DSL Module 101 Transceiver 102 Linetest device 103 Hybrid 104 Transformer 105a Transmission filter 105bDigital/analog converter 106b Analog/digital converter, ADC 106aReception filter, RX filter 107 Echo compensation device FIR echocompensator filter 107a Adder 107b r*4kHz demodulator 108 Sine-wavegenerator 111 and 112 Signal generation devices 113 and 114 Resistors115 Evaluation apparatus Ri Internal resistor 134 and 135 Measurementsignals 131 Specific hardware, low-pass filter 132 Evaluation, register133 Gate, exclusive-NOR gate 134 Transmission signal 135 Received signal141, 142 Mathematical sign formation 143 Phase discriminator 150Quadrature demodulator 210 and 220 Most significant bits (mathematicalsign) 211, 212 Flank 221, 222 Flank and 230 Output signal 231 and 232Flanks 240 Output signal of the low-pass filter

1. A method of qualification of telephone lines as signal conductors forsuitability for data transmissions, comprising: (a) providing, for eachof a plurality of frequencies within a preselected frequency range, atest signal into a telephone line and measuring a reflection signal ofthe test signal, the reflection signal constituting a portion of thetest signal reflected on the input impedance of the telephone line, saidmeasuring including measuring any phase shift in the reflection signalwith respect to the test signal at the respective frequency, (b)determining a first derivative of the phase shift as a function offrequency; (c) determining a second derivative of the phase shift as afunction of the frequency, (d) determining if the second derivative hasat least one mathematical sign change; (e) outputting an indication of asuitability state based on whether at least one mathematical sign changeis determined to exist in the second derivative.
 2. The method ofqualification of telephone lines according to claim 1, wherein each testsignal comprises a sinusoidal AC voltage.
 3. The method of qualificationof telephone lines according to claim 1, wherein, step a) furthercomprises employing a phase discriminator to measure any phase shift inthe reflection signal.
 4. The method of qualification of telephone linesaccording to claim 1, wherein step a) further comprises employing aquadrature demodulator to measure any phase shift in the reflectionsignal.
 5. The method of qualification of telephone lines according toclaim 1, wherein the preselected frequency range is substantially from1.0 kHz to 5.0 kHz.
 6. The method of qualification of telephone linesaccording to claim 1, wherein the plurality of frequencies comprises asequence of frequencies having logarithmic intervals between individualfrequencies of the sequence of frequencies.
 7. The method ofqualification of telephone lines according to claim 1, furthercomprising, prior to step c), averaging individual phase shifts, inorder to smooth them in a profile.
 8. The method of qualification oftelephone lines according to claim 7, further comprising, carrying outmedian formation in the averaging of individual phase shifts.
 9. Themethod of qualification of telephone lines according to claim 8, furthercomprising, prior to step c), generating smoothed phase shifts atregular intervals from each other.
 10. The method of qualification oftelephone lines according to claim 1, wherein the indication of asuitability state comprises an indication that load coils are detectedin the telephone line.
 11. The method of qualification of telephonelines as signal conductors for suitability for data transmissions,comprising: (a) using a DSL modem to provide, for each of a plurality offrequencies within a preselected frequency range, a test signal into atelephone line, (b) measuring a reflection signal of the test signal,the reflection signal constituting a portion of the test signalreflected on the input impedance of the telephone line, said measuringincluding measuring any phase shift in the reflection signal withrespect to the test signal at the respective frequency, (c) determininga first derivative of the phase shift as a function of frequency; (d)determining a second derivative of the phase shift as a function of thefrequency, (e) determining if the second derivative has at least onemathematical sign change; (f) outputting an indication of a suitabilitystate based on whether at least one mathematical sign change isdetermined to exist in the second derivative.
 12. The method accordingto claim 11, wherein step a) further comprises using an existing testmodule of the DSL modem to provide, for each of the plurality offrequencies within the preselected frequency range, the test signal intothe telephone line.
 13. The method according to claim 11, wherein theDSL modem is at least one of the group consisting of: an ISDN modem, aVDSL modem, an ADSL modem, an SHDSL modem and an SDSL modem.
 14. Amethod of qualification of telephone lines as signal conductors forsuitability for data transmissions, comprising: (a) providing, for eachof a plurality of frequencies within a preselected frequency range, atest signal into a telephone line and measuring a reflection signal ofthe test signal, the reflection signal constituting a portion of thetest signal reflected on the input impedance of the telephone line, saidmeasuring including measuring any phase shift in the reflection signalwith respect to the test signal at the respective frequency, (b)determining a first derivative of the phase shift as a function offrequency; (c) filtering the first derivative; (d) determining a secondderivative of the phase shift as a function of the frequency, (e)determining if the second derivative has at least one mathematical signchange; (f) outputting an indication of a suitability state based onwhether at least one mathematical sign change is determined to exist inthe second derivative.
 15. The method of qualification of telephonelines according to claim 14, wherein the preselected frequency range issubstantially from 1.0 kHz to 5.0 kHz.
 16. The method of qualificationof telephone lines according to claim 14, wherein the plurality offrequencies comprises a sequence of frequencies having logarithmicintervals between individual frequencies of the sequence of frequencies.17. The method of qualification of telephone lines according to claim14, wherein step c) further comprises averaging subsets of the firstderivatives in order to smooth them in a profile.
 18. The method ofqualification of telephone lines according to claim 17, furthercomprising, carrying out median formation in the averaging of subsets ofthe first derivatives.
 19. The method of qualification of telephonelines according to claim 18, wherein step c) further comprisesgenerating smoothed phase shifts at regular intervals from each other.20. The method of qualification of telephone lines according to claim14, wherein the indication of a suitability state comprises anindication that load coils are detected in the telephone line.